Fixes #1899, add report that merges all Referrer reports and modified Referrer overview report to support subtables for each referrer type.

Notes:
- modified VisualPHPUnit so Incomplete errors that come from missing expected files have 'Use Processed' link option.
- fixed bug in VisualPHPUnit's 'Use Processed' link (a piwik-specific change).
- Added ability to prepend/append HTML to labels in HTML datatable output by using html_label_prefix/html_label_suffix.
- Fix bug in Piwik_DataTable_Array::mergeSubtables where keyName was not copied in new array.



git-svn-id: http://dev.piwik.org/svn/trunk@7830 59fd770c-687e-43c8-a1e3-f5a4ff64c105